This invention relates generally to a mechanism for saving energy in an air conditioning control system. More particularly, the invention relates to an improvement in a pneumatic control system for optimally controlling a condition, such as air temperature, humidity or enthalpy, in a plurality of zones or regions, although the preferred embodiment is directed to temperature control.
The type of control system to which this invention relates typically includes a plurality of zones or regions in each of which a condition such as temperature is independently controlled. A condition responsive device, such as a thermostat, is located in each zone and adjustably controls the condition to a desired level. A common condition changing medium such as hot or cold water operates through a heat exchanger to change the condition of the air being treated.
The conventional single-duct constant volume air system is illustrated in FIG. 1. The system comprises a return air duct and damper, an outdoor air duct and damper, a cooling coil and fan to discharge the mixture of return air and outdoor air to the zones. In those climates which experience freezing conditions, a preheat coil is placed in front of the cooling coil, and its discharge temperature is designed to be in the neighborhood of 45.degree.-50.degree. F. to protect the cooling coil from freezing. Obviously, in those climates where the outdoor air temperature does not drop below freezing, the preheat coil is not necessary. Also, the ventilation codes of many areas dictate that at all times of occupancy there shall be a minimum amount of fresh air supplied to the zones. This requirement is fulfilled by a minimum outdoor air damper. The rest of the air capacity is supplied through a variable outdoor air damper or through a return air duct damper or by a mixture of outdoor air and return air.
Because of varied heating or cooling loads on the various zones, one zone may require more air treatment than another zone. During winter months, the mixture of return air and minimum outdoor air may be preheated to a temperature to satisfy the zone having the greatest demand, i.e. that zone which will not require any reheat. The result of this operation is that there will always be at least one zone which will not require any reheat.
But during the summer months, the moisture content of the outdoor air is high and this air must be dehumidified in order to maintain comfort in the zones. Thus, the mixture of outdoor air and return air has to be cooled to, for example, 50.degree. F. dry bulb and 50.degree. F. dew point, and the air discharged from the fan, therefore, will be at 50.degree. F. If it is assumed that the zones require an air supply of 60.degree. F., this cold air must be reheated before distribution to all zones.
It is to be noted, however, that if, during the summer months, the temperature of the discharge air is controlled such that the space with the highest cooling demand does not require any reheat, there will result a cost saving since there will always be at least one zone that does not require any reheat as opposed to a system wherein the discharge air must be reheated at each zone to the desired temperature. It is thus desireable to extend the increased savings noted during winter operation to the humid months of summer.